Regex Tester
.Any character except newline\dDigit (0-9)\wWord character (a-z, A-Z, 0-9, _)\sWhitespace^Start of string/line$End of string/line*0 or more+1 or more?0 or 1{n}Exactly n times{n,m}Between n and m times[abc]Character class[^abc]Negated character class(abc)Capturing groupa|bAlternation (a or b)About the Regex Tester
The Regex Tester lets you build, test, and debug regular expressions against sample text in real time, showing exactly which substrings match as you refine the pattern. Regular expressions are a compact mini-language for describing text patterns — character classes, quantifiers, anchors, groups, and alternations — and their power comes at the cost of being notoriously hard to read, which is precisely why an interactive tester is so valuable.
You type a pattern and your test input, and the tool highlights every match, typically surfacing captured groups so you can confirm that your parentheses are isolating the right portions. Flags such as global (find all matches), case-insensitive, and multiline can be toggled to change matching behavior, and many testers report match positions and group indices so you can see precisely where each hit begins and ends.
Developers reach for regex testing when writing input validation, search-and-replace transforms, log parsing, scraping, or routing rules. Testing interactively prevents the classic mistakes of catastrophic backtracking (a pattern that hangs on certain inputs), greedy quantifiers swallowing more than intended, and forgetting to escape special characters like the dot, which matches any character unless escaped.
A practical tip is to build patterns incrementally, adding one piece at a time and confirming the match after each addition rather than writing the whole expression at once. Use non-capturing groups (?:...) when you only need grouping for alternation, and prefer specific character classes over the broad dot to keep patterns fast and predictable. When validating things like email or URL formats, test against both valid and deliberately malformed inputs.
Frequently asked questions
- What do regex flags like g, i, and m do?
- The g (global) flag finds all matches instead of just the first, i (case-insensitive) ignores letter case, and m (multiline) makes the anchors ^ and $ match at the start and end of each line rather than the whole string.
- What is a capturing group?
- A capturing group is a portion of the pattern wrapped in parentheses whose matched text is extracted separately. Groups are numbered left to right and let you pull out specific pieces, such as the year from a date.
- Why does the dot not match a newline?
- By default the dot matches any character except line breaks. Enable the dotall (s) flag, where supported, to make it match newlines too.
- What is catastrophic backtracking?
- It is a performance failure where nested or ambiguous quantifiers cause the engine to try an exponential number of combinations on certain inputs, hanging the match. Avoid it by making quantifiers specific and not overlapping.